Plant virus is one of the major diseases in the crop production. Sometimes, plant virus may impact a catastrophic influence on the agricultural production. Breeding transgenic plants with high antiviral property is a hot point in the biology researches.
The studies in recent years find that the gene silencing mechanism is one of the plant defense mechanisms, through which plants resist the invasion of virus (Covey, S N, 1997, Nature, 385(27): 781-782). The phenomenon of gene silencing was firstly found by Napoli, in conducting the research of the gene chs encoding Chalcone synthase (CHS) in petunia in 1990. (Napoli C, et al. 1990, Plant Cell, 2: 279-289; Van der Krol Ar, et al. 1990, Plant Cell, 2:291-299). Not only the expression of the exogenous chs gene, and but also the expression of the endogenous chs gene is silenced. This phenomenon is called co-suppression. It finds that the phenomenon of gene silencing occurs not only in the transformed gene or transgene, but also in the endogenous gene of the host. The gene is silenced by the induction of the transgene or virus (Ruiz M T, et al., 1998, Plant Cell, 10(6): 937-946; Dalmay T, et al., 2000, Plant Cell, 12(3): 369-379). The phenomenon of gene silencing widely exists in plants, animals, bacteria and fungi (Land, K M. 2001, Trends in Genetics, 17 (7): 379).
Studies find that the “recovery” phenomenon may occur in the plant by transformation of virus gene. It finds that the newly emerging leaves of the transgenic plants infected by a virus have the resistance against the virus. At the same time, the transgene contained in the plant is silenced (Covey, S N, et al. 1997, Nature, 385(27):781-782). Further, when a plant is infected with a virus carrying a plant gene, the corresponding endogenous gene of the plant can also be silenced (Jones L, et al. 1999, Plant Cell, 11(12): 2291-301; Burton R A, et al. 2000, Plant Cell, 12(5): 691-706). When a transgenic plant contains a transgene not derived from viruses and plants, and the transgenic plant is infected with a virus carrying such a gene, the gene can also be silenced. Thus, virus is an object of gene silencing, as well as is an induction factor for producing the gene silencing (Ratcliff, F G. et al. 1999, The Plant Cell, 11, 1207-1215). Plants will activate the gene silencing mechanism when being infected by a virus, making the virus not to propagate in vivo. In this way, the plants will exhibit the immunity or high resistance. The post-transcriptional gene silencing is an immune mechanism for plants to fight against the invasion of virus. (Voinnet, O. 2001, Trends in Genet, 17:449-459; Matzke, M A. et al, 2002, Adv Genet, 46:235-75; Plasterk R H. Science, 2002, 17; 296(5571): 1263-5; Baulcombe D C, Trends Microbiol, 2002, 10(7): 306-8).
The researches of gene silencing mechanism make it possible that the principle of the gene silencing mechanism can be used in breeding high antiviral transgenic plant. A key problem in the use of gene silencing is how to increase the frequency of gene silencing in a target gene. Although one can use a virus vector to effectively induce the gene inactivation, the limitations of virus vector lead that some of the specific genes in a given host cannot be researched and exploited. In addition, although one can use transformation methods to produce gene silencing, the efficiency of producing gene silencing in the natural condition is low. Thus, if a method for improving the inactivation of target gene in a transgenic host may be provided, the transgenic plant with high antiviral property can be created through this method.